Messenger

MESSENGER (Mercury Surface, Space Environment,
Geochemistry, and Ranging) is a probe designed to orbit and study
Mercury, the closest planet to the sun. Planned to launch May 11, 2004, it is the eighth mission of NASA's
Discovery Program.

Intrigue of Mercury

Mercury is a particularly difficult planet to study due in part
to its elusive placement in our sky. Because of its proximity to the sun,
Mercury is rarely present in the Earth's night sky. Using a telescope to
observe Mercury can be quite risky, since catching the sun
through a telescope can be damaging on the eye. The planet is only
visible during 30 to 40 nights of the year. Moreover, Mercury's elliptical
orbit made its trajectory through our sky seemingly unpredictable to astronomers
of yesteryear, and so it has been one of the last planets for science to
understand.

In 1976, NASA's Mariner 10 probe made a major discovery on its flyby of
Mercury. Data suggest the probe encountered a magnetic field much larger
than the Earth's. Although a small planet, only about a third the size of the
Earth, Mercury's iron core is actually relatively much bigger than in the
other inner planets. The foremost objective of the Messenger mission is to
begin to uncover the secret of Mercury's large iron core. Proposed
theories suggest that perhaps Mercury was once a larger planet whose outer crust
was once stripped away from either violent solar activity or a collision with
another world. Another theory states that the center of the early solar
system, then just a disc-like solar nebula, contained drastically different
material or formed differently due to gravitational constraints. A clearer
understanding of Mercury's evolution will enhance our understanding of how other
planetary systems form in the universe. The Messenger mission might yield more
mysteries than it solves, although this seems to be the norm for most space
missions of discovery.

Another objective of the probe is to confirm the theory that water exists
in the puzzling polar regions of Mercury. Because of Mercury's slow
rotation (58 Earth days) and fast orbit around the sun (87 Earth days), an
apparent day on Mercury lasts 176 Earth days. That is, if standing on
Mercury, an apparent day would last months, as the sun slowly rises to high noon
over the course of several months then seems to lazily meander around the sky
before finally heading towards sunset months later. This means that
day-time temperatures on Mercury reach 427°C, while the areas left
dark for many Earth months fall to an astonishing -173°C. The polar
extremities of the planet never see the light of day, and recent data suggests
ice may exist in these parts.

Difficulty of the mission

Inserting an orbiter around Mercury is not an easy task. Huge
amounts of fuel would be needed to slow down a probe to keep it from succumbing
to the sun's powerful draw. Instead, NASA decided to send Messenger on a
five year journey around around the sun, moving slowly inward until finally
inserting itself in an orbit around Mercury in 2009. Messenger will take
advantage of the gravitational stopping power of three Venus flybys en route
to its final destination.

Solar activity, such as solar flare and radiation, is a problem
for space activity around Earth, and the problem is magnified for any mission
that intends to navigate as far inward as Mercury. For this reason, the
Messenger probe's hardware must be more protected than on any other NASA craft
to date. Engineers chose to use a carbon composite frame for the
spacecraft in order to maximize protection from heat. At this proximity to
the sun, the typical aluminum body used in other missions would simply melt.